Isabel is a 5-month-old girl. She has not gained any weight in the last month. She feeds and sleeps irregularly. She also cries frequently and becomes irritable whenever her mother picks her up. Isabel is most likely to be diagnosed with _____.

A) multiple sclerosis
B) Alzheimer's disease
C) failure to thrive syndrome
D) Down syndrome

Isabel's symptoms, including lack of weight gain, irregular feeding and sleeping patterns, frequent crying, and irritability, are indicative of failure to thrive syndrome. This condition is characterized by a child not gaining weight or growing as expected, often due to inadequate nutritional intake, absorption, or utilization, and sometimes associated with emotional neglect or deprivation. Multiple sclerosis and Alzheimer's disease are not relevant to her age or symptoms, and Down syndrome is a genetic disorder typically identified by distinct physical features and developmental delays, not solely by the symptoms described.